1. Field of the Invention
The present invention relates in general to an electrochemical device, and more particularly to improvements in an electrochemical device having a laminar structure, which includes an electrochemical cell employing a planar solid electrolyte body.
2. Discussion of the Prior Art
There has been known an electrochemical device which incorporates an electrochemical cell using a solid electrolyte. For example, such an electrochemical device is used as an oxygen sensor which utilizes zirconia ceramics or other oxygen-ion conductive materials, for determining the concentration of oxygen in an exhaust gas produced by an internal combustion engine of an automotive vehicle. The oxygen concentration is determined according to the principle of an oxygen concentration cell. Also known are electrochemical devices such as detectors or pumps which are adapted to detect hydrogen, nitrogen, carbon dioxide, etc. according to the principle of an oxygen concentration cell, like the oxygen sensor indicated above. Most of such known electrochemical devices use a solid electrolyte material which takes the form of a tubular body having an elongate bore closed at its one end. In view of relatively low productivity and high cost of manufacture of electrochemical devices using such a tubular solid electrolyte body, and from the standpoint of easy assembling of components of the electrochemical device, there has been an increasing trend in the recent years, of using an electrochemical device having a laminar structure wherein electrode layers are formed on a generally planar solid electrolyte body.
An electrochemical cell of such a planar electrochemical device generally constitutes a planar solid electrolyte substrate and at least one pair of electrodes. A common electrochemical device has two electrochemical cells which are superimposed on each other to form an integral laminar structure having an internal space (cavity) that communicates with an external space in which a gas to be measured (measurement gas) exists. The measurement gas is introduced into the internal space, with a predetermined diffusion resistance. One of the electrodes of each eletrochemical cell is exposed to the measurement gas in the internal space. One of the two electrochemical cells is used as a pumping cell which is operable to effect a pumping action to control the concentration of a given component of the measurement gas in the internal space. The other electrochemical cell is used as a sensing cell which is adapted to measure an electromotive force which is induced due to a differential in the concentration of the component to be measured, between the atmosphere in the internal space, and a suitable reference gas.
In such an electrochemical device of a laminar structure having an internal space, one of the pair of electrodes of each of the electrochemical pumping and sensing cells is exposed to the measurement gas in the internal space. The other electrode of the sensing cell is exposed to the reference gas, while the other electrode of the pumping cell is disposed so that it is exposed to the external measurement gas via a suitable porous protective layer. In the case where the measurement gas is "rich-burned" exhaust gases, the electrochemical device indicated above suffers from a problem that the outer pumping electrode of the pumping cell exposed to the exhaust gas tends to be deteriorated by corrosive or reductive gases such as carbon monoxide and hydrocarbon. Further, since the exhaust gases produced, for example, by a motor vehicle are in a chemically non-equilibrium and active state, the contact of such exhaust gases with the outer pumping electrode is likely to cause deterioration of the electrode due to dissolution or precipitation by the exhaust gases. In addition, the exhaust gases which flow at an extremely high velocity will cause vaporization of platinum which is commonly used as a material for the electrodes. The deterioration due to this platinum vaporization is not negligible. In the case of the exhaust gases other than the "rich-burned" exhaust gases, too, the outer pumping electrode tends to be contaminated due to deposition of finely divided particles of various substances contained in the exhaust gases.